Hypothermia is a condition in which body temperature is at a level lower than normal body temperature. Therapeutic induced mild-moderate hypothermia can be beneficial for people suffering stroke, myocardial infarction, cardiac arrest serious head trauma and other conditions involving reduced blood supply. One method for lowering body temperature is to insert a cooling device into an artery of the patient and to internally cool the patient's body by introducing a cooling fluid into the device. A non-invasive technique for lowering body temperature is to externally cool the exterior surface of the patient's body. Such exterior surface cooling could be achieved, for example, by direct contact with a cooling fluid, such as by immersing the patient's body in the cooling fluid or by directing the flow of the cooling fluid around the patient's body. The cooling fluid could be, for example, cool water or cool air. Another technique for external surface cooling is to apply a contact-cooling pad to the exterior surface of the patient and to circulate a cooling fluid, such as water or an aqueous solution, through the contact pad to cool the patient.
For therapeutic purposes, it is often desirable for the mild-moderate hypothermia to be induced very quickly. With endovascular cooling, heat is removed directly from blood flowing through blood vessels. Blood with reduced temperature moves through blood vessels to cool other parts of the body. With exterior surface cooling, heat is removed across the patient's skin. Cooling of the skin increases conduction of heat from deeper within the body, thereby cooling internal body tissue. Blood moving through blood vessels in a cooled portion of the body is also cooled, and distribution of that cooled blood to other parts of the body thereby contributes to cooling other parts of the body.
Quick inducement of hypothermia requires that the patient's body temperature be rapidly reduced to the desired level, and involves a high rate of transfer of heat from the body. Impediments to inducing hypothermia include the patient's thermoregulatory responses to cooling. Shivering is a common thermoregulatory response that, in some cases, can increase body heat production to as much as 600% above basal levels. Anti-shivering drugs, and particularly meperidine, have been administered prior to or during active cooling to help suppress the shivering response. Such pharmacological treatment to suppress shivering is often successful, resulting in more rapid lowering of the patient's body temperature to more quickly induce a desired degree of hypothermia, reducing patient tiring attendant to shivering, and also reducing patient discomfort associated with shivering.